In the aircraft industry various components of an aircraft wing are riveted together by slug rivets. A slug rivet is a rivet formed from a slug that does not have a preformed head and which has both ends deformed during riveting. Because of the large number of rivets utilized to produce a single aircraft wing, and also because of the requirements of the virtually indefinite life of the rivets, much attention has been given in the industry to various methods and apparatus for riveting. One riveting method and apparatus which has recently been proposed is shown in U.S. Pat. No. 4,908,928. When practicing the method and apparatus disclosed in the aforementioned patent the upper and lower riveting rams are mounted by cylinder assemblies in a generally C-shaped frame such as that shown schematically in FIG. 1 of U.S. Pat. No. 4,864,713. According to the method and apparatus U.S. Pat. No. 4,908,928 after the upper and lower riveting rams have been snugged against the rivet, which has previously been inserted into aligned apertures in the workpieces, fluid is simultaneously introduced in the cylinders behind each of the upper and lower riveting rams to cause the riveting rams to move together at substantially the same with respect to the cylinder. As the upper and lower riveting rams are moving at the same rate with respect to the cylinder there is little movement of the workpiece during riveting. In other words, the workpieces will not move, or will barely move, relative to a fixed work plane. Movement of the workpieces with respect to a fixed work plane is known in the industry as a "wink". If the workpieces are winked or moved during the squeeze cycle of a riveting process they will have a tendency to oscillate before returning to their original position. This oscillation could delay the next rivet forming operation or any other subsequent operation. If there is limited movement of the workpieces during riveting even greater uniformity of the bulging of the rivet may be achieved which is desirable for rivet fatigue life cycles. The limited movement of the workpiece during slug forming permits clamping the workpiece into a rigid fixture which guarantees the accuracy of the workpiece geometry. Since it is not required that the riveter tooling lift the workpiece to cause wink, there is a reduced chance that the tooling will mar the surface of the workpiece.
When using the machine of U.S. Pat. No. 4,908,928 many of the foregoing objectives have been achieved. However, the foregoing machine includes a midpoint supported C-frame which can be shifted vertically, and which can also be rocked and tilted so that the riveting rams may be positioned perpendicular to the surface of the upper workpiece which is held in a stationary manner. However, in some instances, it is desirable to support the wing for vertical and tilting movements while supporting the riveting rams only for X and Y movements in the horizontal plane. When this form of apparatus is utilized the riveting rams are supported on a large C-shaped frame which is supported at a lower portion of the C-shaped frame. Accordingly, it can be appreciated that when using this form of device that as the upper riveting ram moves downwardly against a rivet which is simultaneously being squeezed by a lower riveting ram that the upper frame may deflect slightly, for example 50/1000 of an inch. In some applications this much movement is undesirable and therefore it is desirable that a method and apparatus be developed which will compensate for frame deflection.
It has also been found in the design shown in U.S. Pat. No. 4,908,928 that when the upper and lower rams have been brought into snug engagement with a slug rivet at the commencement of the slug upsetting operation that the pressures bearing upon the pistons for the upper and lower riveting rams are not equal but that, in fact, the pressure on the lower ram is greater than the upper ram. At the commencement of the upsetting operation therefore there is a slight tendency for the upper ram to be deflected upwardly and it is desirable that this be avoided.